Conventionally, WC-based cemented carbides, TiCN-based cermets, cBN sintered materials, and the like are well known as tool materials, but in recent years, a tool material formed of a composite member has been proposed instead of forming a tool material with a single material.
For example, Japanese Unexamined Publication No. 2009-241236 proposes a bonded body including a cermet sintered material as a first bonded material 1 and a cBN sintered material or a diamond sintered material as a second bonded material 3, in which bonding is performed between the first bonded material and the second bonded material via a bonding material 2 which does not generate a liquid phase when a temperature is less than 1000° C., by electrically heating while applying a pressure of 0.1 MPa to 200 MPa, and since a bonding layer does not decrease in bonding strength even at a high temperature exceeding a temperature at which a brazing material generates a liquid phase during cutting, the bonded body obtained by the method described above is suitable for high-speed cutting tools or CVD coated cutting tools.
Further, Japanese Unexamined Publication No. 2012-111187 proposes obtaining a bonded body for a cutting tool or the like having a high bonding strength, in a bonded body including a cemented carbide sintered material as a first bonded material 1 and a cBN sintered material as a second bonded material 2, in which bonding is performed at least two surfaces including a back surface and a bottom surface of the second bonded material between the first bonded material and the second bonded material via a bonding material 3 containing titanium (Ti), by forming a titanium nitride (TiN) compound layer having a thickness of 10 to 300 nm at an interface between the second bonded material and a bonding material, and by making a thickness of the bonding layer of the back surface less than a thickness of the bonding layer of the bottom surface.
Further, Japanese Unexamined Patent Application, First Publication No. 2014-131819 proposes enhancing bonding strength of a composite body, in a composite body including a cBN material containing cBN at 20 to 100% by mass, and a hard alloy formed of a hard phase at 50 to 97% by mass formed of at least one selected from the group consisting of carbides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, carbonitrides thereof and mutual solid solutions thereof, and a binder phase as the balance, at 3 to 50% by mass of which a main component is at least one selected from the group consisting of Co, Ni, and Fe, by providing a bonding layer between the cBN sintered material and the hard alloy, by forming the bonding layer with a ceramic phase and a metal phase, and by setting a thickness of the bonding layer to 2 to 30 μm.
Although the composite materials proposed in Japanese Unexamined Publication No. 2009-241236, Japanese Unexamined Publication No. 2012-111187 and Japanese Unexamined Publication No. 2014-131819 described above or cutting tools made thereof exhibit a certain level of performance in cutting under normal conditions, in heavy cutting condition, for example, a high-feed rate and large depth of cut, in which a high load is exerted on a cutting edge, bonding strength cannot be said to be sufficient and there is a likelihood of breakage from the bonding portion.
Therefore, a composite member having even higher bonding strength of a bonding portion which does not cause breakage at the bonding portion even under heavy cutting conditions in which a high load is exerted on the cutting edge, and a cutting tool made thereof, are desired.
In order to solve the problems of the conventional composite member and a cutting tool made thereof, in a composite member formed of a WC-based cemented carbide and a WC-based cemented carbide and in a cutting tool made of the composite member, for example, in a cutting tool in which a cutting edge portion formed of a composite sintered material obtained by bonding a WC-based cemented carbide (backing material) simultaneously with sintering of a cBN sintered material at the time of ultrahigh pressure high temperature sintering and a WC-based cemented carbide tool body (base body) are bonded via a bonding member, as a result of intensive research on measures to improve the bonding strength of the bonding portion, the inventors of the present application made the following discoveries.
One WC-based cemented carbide member (hereinafter referred to as “WC-based cemented carbide member A”) and the other WC-based cemented carbide member (hereinafter referred to as a “WC-based cemented carbide member B”) are bonded via a bonding member made of a Ti foil. In the composite member in which the WC-based cemented carbide member A and the WC-based cemented carbide member B are bonded via a bonding layer, a first A layer including a TiC phase and a metal W phase is formed as the bonding layer adjacent to the WC-based cemented carbide member A, a second A layer including a TiCo phase and a metal Ti phase is formed adjacent to the first A layer, and, similarly in the WC-based cemented carbide member B, a first B layer including a TiC phase and a metal W phase is formed as the bonding layer adjacent to the WC-based cemented carbide member B, a second B layer including a TiCo phase and a metal Ti phase is formed adjacent to the first B layer, and, furthermore, a residual Ti layer is formed in a central region of the bonding layer sandwiched between the second A layer and the second B layer. In the composite member in which the WC-based cemented carbide member A, the first A layer, the second A layer, the residual Ti layer, the second B layer, the first B layer, and the WC-based cemented carbide member B are bonded in this order, when an area ratio of the TiC phase and a layer thickness in the first A layer and the first B layer, and an area ratio of the TiCo phase and a layer thickness in the second A layer and the second B layer are maintained within an appropriate range, adhesion strength and bonding strength between the WC-based cemented carbide member and the bonding layer can be enhanced.
Further, it has been found that the adhesion strength and bonding strength between the WC-based cemented carbide member and the bonding layer can be further enhanced when the area ratio occupied by the TiC phase in the first A layer increases gradually from the WC-based cemented carbide member A side toward the second A layer side and when the area ratio occupied by the TiC phase in the first B layer increases gradually from the WC-based cemented carbide member B side toward the second B layer side.
Therefore, it has been found that when the composite member is used as a material for a cutting tool, occurrence of breakage from the bonding portion can be prevented and excellent cutting performance can be exhibited for long-term usage, even when it is used for heavy cutting of steel or cast iron in which a high load is exerted on a cutting edge.